Rule-based systems are increasingly used to determine diagnoses and treatments in a variety of settings, e.g., in medicine, manufacturing, business, government. For example, in a clinical setting, adhering to a rule-based system may ensure that a patient with a severe allergy to penicillin is not given a drug in the same family as penicillin, or that a 40 year old woman is reminded to get an annual pap smear for cervical cancer screening. Typically, technical personnel must create and maintain the systems that encode and enforce these rules, but domain experts (e.g., medical doctors, shop foremen, business executives, and government supervisors) must write and maintain the content of these rules. Many systems are not user-friendly enough for domain experts to use effectively—both initially and over time to keep a rule-based system up-to-date. Indeed, having an out-of-date rule can pose a safety and legal threat to a health care provider using the out-of-date rule in a decision support system.